Kumar held positions, including additional secretary in the Ministry of Electronics and Information Technology and director general of the National Informatics Centre.

He was part of the team that implemented Digital India Initiatives in 2014 which included Unified Payments Interface, Aadhaar, MyGov.in, Government e Marketplace, and Jeevan Pramaan.

As a former Defence Secretary, Kumar worked in the corporatization of the Ordnance Factories Board, a 200-year-old institution employing 80,000 people.

He is an active writer for Indian national dailies and maintains a monthly column in Business Standard titled "Off the Grid."

Kumar held the post of Joint Secretary, Ministry of Electronics and Information Technology and additional charge of Director General at National Informatics Centre from 2010 to 2014.

[8] He served as Defence Production Secretary till 23 August 2019 and was succeeded by Subhash Chandra, an IAS officer of Karnataka cadre (1986 batch).

[20] Dr Ajay Kumar, the Defence Secretary, commissioned the Indian Coast Guard Ship (ICGS) Saksham, the fifth of the 105-metre Offshore Patrol Vessels (OPVs) class.